This invention relates to display units.
The invention is more particularly concerned with display units in which a display representation is provided by energization of light-emitting elements together with illumination of light-reflective regions.
Many forms of instruments and other display units include both light-emitting elements, such as light-emitting diodes, and reflective regions such as printed markings. In normal daylight, the light-emitting elements must be sufficiently brightly energized to ensure that they are easily readable, while ambient light provides sufficient illumination of the reflective markings or other elements.
In subdued light, the level of energization of the light-emitting elements is reduced, while illumination, in addition to ambient light, may be required of the reflective markings to make them clearly visible.
In night conditions, greater illumination of the markings is required while a lower brightness level is needed for the light-emitting elements.
For some applications, such as on aircraft flight decks, the control of apparent brightness of the display is very important since both the light-emitting and light-reflective elements must always be clearly visible in conditions that vary from bright sunlight to pitch darkness. It is also important that, in darkness conditions, the brightness of the display does not distract the user from other displays, controls or from faintly illuminated objects outside the aircraft.
Conventional display units include a light detector responsive to ambient light which is coupled to the drive circuit of the light-emitting elements, so that the brightness of the elements is varied directly in response to ambient light levels. In this way, the light-emitting elements are energized to a higher level at bright ambient light conditions than in dark conditions. The level of illumination of the reflective markings is controlled manually by the user as he desires, so that the markings are more brightly illuminated in dark conditions.
This previous arrangement, however, cannot readily provide a clearly visible display especially at low ambient light levels, because the relatively high level of illumination required for the reflective markings will mask the relatively low brightness of the light-emitting elements produced automatically. This generally results in the user compromising by setting a lower than ideal brightness for the reflecting markings, or by the display being arranged to produce a higher than desirable brightness level for the light-emitting elements.
In some displays, it may be possible to locate the light detector so that it receives both ambient light and light from the source used to illuminate the reflective elements. In such an arrangement, a degree of compensation is automatically provided when ambient light levels are low, and when light levels from the illuminating source are high.
However, in many displays it is not possible to do this, such as when the reflective elements are markings on a face of a transparent plate that is edge-illuminated through the thickness of the plate.